1. Field of the Invention
The present invention relates to a process for producing an electric circuit board in which an electric circuit pattern is formed on the surface of an insulating substrate of synthetic resin simultaneously with the molding thereof.
2. Related Background Art
Conventionally an electric circuit pattern is formed on the surface of a molded product such as of resin either by a subtractive method in which a conductor such as a copper foil is adhered to a molded product and is etched in unnecessary portions, or by a full-additive method in which the surface of the molded product is subjected to a base treatment for improving adhesion strength, followed by addition of a catalyst to form plating nuclei and electroless plating, or a semi-additive method in which electroplating is employed in combination with electroless plating.
Also, there has been developed a method of printing electroconductive paint on the molded product, or a method of applying a sheet printed with electroconductive paint on the molded product and thermally transferring the paint thereto.
However, all of these methods requires, after the molding of a product, a step of forming an electric circuit pattern on the surface thereof.
The present inventors have therefore proposed a process of producing an electric circuit board having a circuit pattern on an insulating substrate, by charging a transfer sheet, bearing a circuit pattern on a sheet substrate in a metal mold for forming an insulating resin substrate and filling the mold with resin thereby transferring the circuit pattern of the transfer sheet onto the surface of the insultaing substrate simultaneously with the molding thereof. This process eliminates necessity of a separate step for forming the circuit pattern as a post-treatment and enables the very efficient manufacture of the electric circuit board.
However, the conventional transfer sheet employable in this process is prepared by printing, on a sheet substrate, a circuit pattern with conductive paint containing metal fillers such as silver, copper, nickel etc., and such conductive paint containing such filler shows a high electrical resistance. Therefore, the electric circuit to be produced by use of such a transfer sheet is associated with certain limitations, and such a process is not suitable for electric circuit boards requiring the application of a large current or those calling for formation of fine circuit patterns with narrower width, due to heat generation or current loss.